1. Field of the Invention
In the United States, Residential space heating consumes 82 billion kWh of electricity, 2,870 billion cubic feet of natural gas, 5,251 million gallons of fuel oil, 127 million gallons of kerosene, and 3,521 millions gallons of LPG annually. Commercially, building heating accounts for 1.04 trillion kWh electricity, 615 billion kWh natural gas, and 67 billion kWh of fuel oil. 70% of the energy produced in the US last year was obtained from combustion of fossil fuels (coal, natural gas, and oil) and each kWh used requires 3 kWh in fossil fuel energy at the generator. Any improvement in the efficiency and reliability of installed heating systems, even in small percentages, has a significant impact on energy consumption and emissions of greenhouse gasses.
Heat pumps play an important role in achieving energy savings in heating and cooling. A heat pump is a relatively simple thermodynamic system whose purpose is to transport heat from a colder environment (e.g. from the outdoors) to a warmer environment (the indoor space). When used in reverse, the same system becomes an air conditioner, which transfers of from the cooler indoor space to the warmer outdoor environment. To achieve this transport of heat, the heat pump uses electricity or mechanical work to drive a thermodynamic cycle, comprising a working gas (refrigerant), a compressor 1, a condenser 2, an expansion valve 3 and an evaporator 4 as seen in FIG. 1. Arrows in the figure indicate the direction of flow of the working gas.
2. Description of Related Art
The performance of an air source heat pump degrades when it operates at either very low or very high temperatures. This performance degradation is due to an increase in irreversibilities during the refrigerant compression process, a reduction in the refrigerant mass flow, and a deterioration of the heat transfer capacity in the heat exchangers. If most of the high end, commercially available heat pump systems achieve coefficients of performance (COP) as high as 4-6 (i.e. for each kW of work input, 4-6 kW of heat is transferred to the heated space) when operating at nominal ambient conditions of 45-47° F. or above, their coefficient of performance drops to 1.5-1.8 at 15° F. To supplement the loss of efficiency and of heating capacity at low ambient (cold source) temperatures, most of these systems are equipped with electrical or gas fired heaters/furnaces. Currently there are no heat pumps systems offered commercially that operate efficiently at temperatures lower than 15° F.
Air source heat pumps have the possibility to operate beyond their nominal ranges while preserving cycle efficiency by modifying their system configurations (Kim (2001), Bertsch and Groll (2008), Wang et al. (2009), and Heo et al. (2010a)). The merits of several modified refrigeration cycles are summarized by Heo et al. (2010b). FIG. 2A-D shows schematic diagrams of systems used in four methods for improving the efficiency of vapor compression refrigeration cycles. FIG. 2A shows a Flash-Tank Vapor Injection (FTVI) system; FIG. 2B shows a Flash-Tank and Sub-Cooler Vapor Injection (FTSC) system; FIG. 2C shows a Sub-cooler Vapor Injection (SCVI) system; and FIG. 2D shows a Double Expansion Sub-cooler Vapor Injection (DESC) system. All of the systems comprise a compressor 1, a condenser 2, expansion valves 3, and an evaporator 4. The FTVI system additionally comprises a flash tank located between the condenser 2 and evaporator 4 and an injection valve 6 controlling flow of working gas from the flash tank 5 to the compressor 1. The FTSC system, like the FTVI system, additionally comprises a flash tank located between the condenser 2 and evaporator 4 but also includes a subcooler 7 between the flash tank 5 and the evaporator 4 and between the flash tank 5 and the compressor 1. The SCVI and DESC systems comprise a receiver 8 and a subcooler 7 in series between the condenser 2 and evaporator 4 and an injection valve 6 controlling flow of working gas from the flash tank 5 to the compressor 1.
These cycles improve the efficiency of a regular vapor compression cycle by increasing the amount of heat transfer at constant temperature (in the two phase and liquid state) and reducing the amount of work required to compress the refrigerant vapor between the two isobars of the cycle. The difference between the regular vapor compression cycles (dashed line) and vapor-injected cycles (grey line) corresponding to the systems shown in FIG. 2A-D are shown in the pressure—enthalpy diagrams in FIG. 3 A-D. The performance of each cycle is directly proportional to the area enclosed by the cycle under the saturation curve (curved black line).
The modifications to existing heat pumps suggested by the above-referenced articles suffer from several drawbacks. For example, there is insufficient heat output as the required heat increases whereas the heat pump capacity decreases mainly due to lower refrigerant mass flow rates delivered by the compressor at high pressure ratios. High compressor discharge temperature is caused by low suction pressure and high pressure ratio across the compressor. COP decreases rapidly for the high pressure ratios necessary for heating at low ambient temperature conditions. Heat pumps designed for low ambient temperature conditions usually have capacities that are too large at medium ambient temperatures. This requires cycling of the heat pump on and off at higher ambient temperatures in order to reduce the heating capacity. Transient effects associated with cycling leads to a lower efficiency relative to steady-state operation. The FTVI cycle may experience flooding in the compressor at high speeds due to the difficulty of accurately controlling the amount of vapor injection.